Network Protocols and Vulnerabilities

1. Spring 2009 CS 155 Network Protocols and Vulnerabilities Dan Boneh

2. Outline • Basic Networking: • How things work now plus some problems • Some network attacks • Attacking host-to-host datagram protocols • TCP Spoofing, … • Attacking network infrastructure • Routing • Domain Name System

3. Internet Infrastructure • Local and interdomain routing • TCP/IP for routing, connections • BGP for routing announcements • Domain Name System • Find IP address from symbolic name (www.cs.stanford.edu) Backbone ISP ISP

4. TCP Protocol Stack Application protocol Application Application TCP protocol Transport Transport Network IP Network IP protocol IP protocol Link Network Access Link Data Link Data Link

5. Data Formats TCP Header Application Application message - data message Transport (TCP, UDP) segment TCP data TCP data TCP data Network (IP) packet IP TCP data Link Layer frame ETH IP TCP data ETF IP Header Link (Ethernet) Header Link (Ethernet) Trailer

6. IP Version Header Length Type of Service Total Length Identification Flags Fragment Offset Time to Live Protocol Header Checksum Source Address of Originating Host Destination Address of Target Host Options Padding IP Data Internet Protocol • Connectionless • Unreliable • Best effort • Notes: • src and dest ports not parts of IP hdr

7. Packet 121.42.33.12 Source 132.14.11.51 Destination IP Routing • Internet routing uses numeric IP address • Typical route uses several hops Meg Office gateway Tom 121.42.33.12 132.14.11.1 ISP 132.14.11.51 121.42.33.1

8. IP Protocol Functions (Summary) • Routing • IP host knows location of router (gateway) • IP gateway must know route to other networks • Fragmentation and reassembly • If max-packet-size less than the user-data-size • Error reporting • ICMP packet to source if packet is dropped • TTL field: decremented after every hop • Packet dropped f TTL=0. Prevents infinite loops.

9. Problem: no src IP authentication • Client is trusted to embed correct source IP • Easy to override using raw sockets • Libnet: a library for formatting raw packets with arbitrary IP headers • Anyone who owns their machine can send packets with arbitrary source IP • … response will be sent back to forged source IP • Implications: (solutions in DDoS lecture) • Anonymous DoS attacks; • Anonymous infection attacks (e.g. slammer worm)

10. UDP User Datagram Protocol • Unreliable transport on top of IP: • No acknowledgment • No congenstion control • No message continuation

11. TCP Transmission Control Protocol • Connection-oriented, preserves order • Sender • Break data into packets • Attach packet numbers • Receiver • Acknowledge receipt; lost packets are resent • Reassemble packets in correct order Book Mail each page Reassemble book 1 19 5 1 1

12. TCP Header Source Port Dest port SEQ Number ACK Number U R G A C K P S H P S R S Y N F I N TCP Header Other stuff

13. Review: TCP Handshake C S SNCrandC ANC0 SYN: Listening SNSrandS ANSSNC Store SNC , SNS SYN/ACK: Wait SNSNC+1 ANSNS ACK: Established Received packets with SN too far out of window are dropped

14. Basic Security Problems 1. Network packets pass by untrusted hosts • Eavesdropping, packet sniffing • Especially easy when attacker controls a machine close to victim 2. TCP state can be easy to guess • Enables spoofing and session hijacking 3. Denial of Service (DoS) vulnerabilities • DDoS lecture

15. 1. Packet Sniffing • Promiscuous NIC reads all packets • Read all unencrypted data (e.g., “wireshark”) • ftp, telnet (and POP, IMAP) send passwords in clear! Eve Network Alice Bob • Sweet Hall attack installed sniffer on local machine Prevention: Encryption (next lecture: IPSEC)

16. 2. TCP Connection Spoofing • Why random initial sequence numbers? (SNC , SNS ) • Suppose init. sequence numbers are predictable • Attacker can create TCP session on behalf of forged source IP • Breaks IP-based authentication (e.g. SPF, /etc/hosts ) TCP SYN Server attacker Victim srcIP=victim SYN/ACKdstIP=victim SN=server SNS ACK srcIP=victim AN=predicted SNS server thinks command is from victim IP addr command

17. Example DoS vulnerability [Watson’04] • Suppose attacker can guess seq. number for an existing connection: • Attacker can send Reset packet to close connection. Results in DoS. • Naively, success prob. is 1/232 (32-bit seq. #’s). • Most systems allow for a large window of acceptable seq. #’s • Much higher success probability. • Attack is most effective against long lived connections, e.g. BGP

18. Random initial TCP SNs • Unpredictable SNs prevent basic packet injection • … but attacker can inject packets after eavesdropping to obtain current SN • Most TCP stacks now generate random SNs • Random generator should be unpredictable • GPR’06: Linux RNG for generating SNs is predictable • Attacker repeatedly connects to server • Obtains sequence of SNs • Can predict next SN • Attacker can now do TCP spoofing (create TCP session with forged source IP)

19. Routing Vulnerabilities

20. Routing Vulnerabilities • Common attack: advertise false routes • Causes traffic to go though compromised hosts • ARP (addr resolution protocol): IP addr -> eth addr • Node A can confuse gateway into sending it traffic for B • By proxying traffic, attacker A can easily inject packets into B’s session (e.g. WiFi networks) • OSPF: used for routing within an AS • BGP: routing between ASs • Attacker can cause entire Internet to send traffic for a victim IP to attacker’s address. • Example: Youtube mishap (see DDoS lecture)

21. Interdomain Routing earthlink.net Stanford.edu BGP Autonomous System connected group of one or more Internet Protocol prefixes under a single routing policy (aka domain) OSPF

22. BGP overview • Iterative path announcement • Path announcements grow from destination to source • Packets flow in reverse direction • Protocol specification • Announcements can be shortest path • Not obligated to use announced path

23. 3 2 7 2 7 3 2 6 5 2 6 5 2 6 5 5 2 7 6 5 7 2 6 5 2 7 7 6 2 7 7 2 6 5 5 BGP example [D. Wetherall] • Transit: 2 provides transit for 7 • Algorithm seems to work OK in practice • BGP is does not respond well to frequent node outages 3 4 1 8 2 5 6 7

24. Issues • Security problems • Potential for disruptive attacks • BGP packets are un-authenticated • Attacker can advertise arbitrary routes • Advertisement will propagate everywhere • Used for DoS and spam (detailed example in DDoS lecture) • Incentive for dishonesty • ISP pays for some routes, others free

25. Domain Name System

26. DNS Domain Name System • Hierarchical Name Space root edu com uk net org ca stanford cmu mit ucb wisc cs ee www

27. Hierarchical service Root name servers for top-level domains Authoritative name servers for subdomains Local name resolvers contact authoritative servers when they do not know a name DNS Root Name Servers

28. DNS Lookup Example root & edu DNS server www.cs.stanford.edu www.cs.stanford.edu NS stanford.edu stanford.edu DNS server Local DNS resolver NS cs.stanford.edu Client A www=IPaddr cs.stanford.edu DNS server DNS record types (partial list): - NS: name server (points to other server) - A: address record (contains IP address) - MX: address in charge of handling email - TXT: generic text (e.g. used to distribute site public keys (DKIM) )

29. Caching • DNS responses are cached • Quick response for repeated translations • Useful for finding servers as well as addresses • NS records for domains • DNS negative queries are cached • Save time for nonexistent sites, e.g. misspelling • Cached data periodically times out • Lifetime (TTL) of data controlled by owner of data • TTL passed with every record

30. DNS Packet • Query ID: • 16 bit random value • Links response to query (from Steve Friedl)

31. Resolver to NS request

32. Response to resolver Response contains IP addr of next NS server (called “glue”) Response ignored if unrecognized QueryID bailiwick checking: response is cached if it is within the same domain of query (i.e. a.com cannot set NS for b.com)

33. Resolver response to client final answer

34. Basic DNS Vulnerabilities • Users/hosts trust the host-address mapping provided by DNS: • Used as basis for many security policies: Browser same origin policy, URL address bar • Obvious problems • Interception of requests or compromise of DNS servers can result in incorrect or malicious responses • e.g.: hijack BGP route to spoof DNS • Solution – authenticated requests/responses • Provided by DNSsec … but no one uses DNSsec

35. DNS cache poisoning (a la Kaminsky’08) • Victim machine visits attacker’s web site, downloads Javascript a.bank.com QID=x1 Query: a.bank.com user browser local DNS resolver ns.bank.com IPaddr 256 responses: Random QID y1, y2, … NS bank.com=ns.bank.com A ns.bank.com=attackerIP attacker wins if j: x1 = yj response is cached and attacker owns bank.com attacker

36. If at first you don’t succeed … • Victim machine visits attacker’s web site, downloads Javascript b.bank.com QID=x2 Query: b.bank.com user browser local DNS resolver ns.bank.com IPaddr 256 responses: Random QID y1, y2, … NS bank.com=ns.bank.com A ns.bank.com=attackerIP attacker wins if j: x2 = yj response is cached and attacker owns bank.com attacker success after  256 tries (few minutes)

37. Defenses • Increase Query ID size. How? a. Randomize src port, additional 11 bits Now attack takes several hours b. Ask every DNS query twice: • Attacker has to guess QueryID correctly twice (32 bits) • Apparently DNS system cannot handle the load

38. Pharming • DNS poisoning attack (less common than phishing) • Change IP addresses to redirect URLs to fraudulent sites • Potentially more dangerous than phishing attacks • No email solicitation is required • DNS poisoning attacks have occurred: • January 2005, the domain name for a large New York ISP, Panix, was hijacked to a site in Australia. • In November 2004, Google and Amazon users were sent to Med Network Inc., an online pharmacy • In March 2003, a group dubbed the "Freedom Cyber Force Militia" hijacked visitors to the Al-Jazeera Web site and presented them with the message "God Bless Our Troops"

39. DNS Rebinding Attack Read permitted: it’s the “same origin” www.evil.com? 171.64.7.115TTL = 0 192.168.0.100 [DWF’96, R’01] <iframe src="http://www.evil.com"> DNS-SEC cannot stop this attack Firewall ns.evil.com DNS server www.evil.com web server corporate web server 171.64.7.115 192.168.0.100

40. DNS Rebinding Defenses • Browser mitigation: DNS Pinning • Refuse to switch to a new IP • Interacts poorly with proxies, VPN, dynamic DNS, … • Not consistently implemented in any browser • Server-side defenses • Check Host header for unrecognized domains • Authenticate users with something other than IP • Firewall defenses • External names can’t resolve to internal addresses • Protects browsers inside the organization

41. Summary • Core protocols not designed for security • Eavesdropping, Packet injection, Route stealing, DNS poisoning • Patched over time to prevent basic attacks (e.g. random TCP SN) • More secure variants exist (next lecture) : IP -> IPsec DNS -> DNSsec BGP -> SBGP